1. Field of the Invention
The present invention relates to a serial printer device in which the coil of a print head is prevented from being burned by detecting the temperature of the print head switching the printing mode of the print head in accordance with the detected temperature.
2. Description of the Relevant Art
There is known from Japanese Patent Publication No. 1-56916 (published on Dec. 1, 1989) a serial printer which includes: a temperature detecting element disposed in or around a print head for detecting the temperature of the print head; and control means for switching a bidirectional printing mode to a unidirectional print mode when the detected temperature detected by the temperature detecting element exceeds a predetermined value. In case the bidirectional printing operation is carried out at a high speed, the print head has its temperature raised gradually. In the aforementioned serial printer, the coil of the print head is prevented from being burned even if the temperature of the print head rises, by switching the bidirectional print mode to the unidirectional printing mode.
In general, when a high-density printing operation, in which the frequency of actions of the armature of the print head per one line is high (for example, a printing to paint out one line) is repeated, the temperature of the coil in the print head rises. As a matter of fact, the temperature rising rate of the coil is so high that the temperature detecting element in the print head cannot follow the rise, to cause a considerable difference between the temperature of the coil and the detected temperature of the temperature detecting element. In the prior art, therefore, the temperature to be set for switching the bidirectional printing mode to the print stopping mode or the unidirectional printing mode when the coil temperature rises is set to a far lower level than the temperature at which the coil is actually burned. Specifically, the set value to be compared with the detected value of the temperature detecting element is set to a far lower level than the limit temperature, below which the coil is not burned, so that the printing mode is switched to the print stopping mode or the unidirectional printing mode when the detected value of the temperature detecting element reaches said set value. The aforementioned set value to be compared with the detected value of the temperature detecting element is so set to the limit temperature, below which the coil is not burned when in the high-density printing operation of high heat generation of the coil that the coil may not be burned during the various printing operations of the printer. In the actual printing operation, the printing density per line may be high or low. In case of repeating the low-density printing operation, the temperature rising rate of the coil is gentle and therefore the difference between the detected value of the temperature detecting element and the actual coil temperature is small. In such case, the detected value of the temperature detecting element may reach the set value although the actual coil temperature is far from the coil unburned limit temperature. As a result, the switching control to the print stopping mode or the unidirectional print mode is effected to cause a problem that the throughput time, i.e., the time period for printing a predetermined amount is elongated.
In order to solve this problem, it is sufficient to enhance the response of the temperature detecting element, i.e., to reduce the difference between the coil temperature and the temperature detected by the temperature detecting element. For this, there can be conceived a method of filling a thermal conducting agent or the like between the temperature detecting element and the coil. However, this method leads to another problem that the production cost is raised.
In general, moreover, the carriage speed for the unidirectional printing operation is set to be higher in the non-printing return than in the printing operation. This of carriage speed becomes larger in the high-density printing mode, and therefore the time period for cooling the coil of the print head is so short that little effect is obtained for suppressing the rise of the coil temperature.